1. Field of the Invention
This invention relates to a multi-layered white top paperboard and to a method for making same. More particularly, this invention relates to multi-layered paperboard having a base layer(s) of unbleached fiber and a highly filled white top layer having a coating to achieve over 65% GE brightness and exceptional uniformity and to a method for making same.
2. The Prior Art
For many years filled white topped papers have been produced. Thus, it is conventional to add fillers to pulp fibers when making paper to improve the opacity, whiteness, printability of the paper and to reduce the cost of the paper because fillers are normally cheaper than the cellulose fibers which they replace. The problem is that fibers are expensive and retention of sufficient fillers is difficult.
It is also known to produce highly filled sheets of paper with water-insoluble, organic polymers to help retain the finely-divided, substantially water-insoluble, non-fibrous, inorganic filler on the fiber. However, the retention of fillers in the sheet during formation is a significant problem. Also, it is known that increasing filler content progressively reduces sheet strength because the filler particles interfere with the potential development of fiber-to-fiber bonds and occupy spaces that might otherwise have contained fibers.
In the paperboard area, current methods of manufacturing white top paperboard and folding carton boards employ either bleached pulp fibers in a separately formed top ply or multiple stages of coatings with pigments and binders. These approaches have the disadvantages of higher cost materials, negative environmental impact for chemically bleached fiber and large capital costs and costs for multiple forming sections and multiple coating and re-drying equipment for the coated process.